Integrators for delta-sigma modulators

ABSTRACT

Methods, systems and devices are disclosed. Among the disclosed devices is an electronic device that, in certain embodiments, includes a plurality of memory elements or imaging elements connected to a bit-line and a delta-sigma modulator connected to the bit-line. The delta-sigma modulator may include an integrator having a differential amplifier.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No. 12/886,377, which was filed on Sep. 20, 2010, now U.S. Pat. No. 8,102,295, which issued on Jan. 24, 2012, which is a divisional of U.S. patent application Ser. No. 11/818,998, which was filed on Jun. 15, 2007, now U.S. Pat. No. 7,817,073, which issued on Oct. 19, 2010.

BACKGROUND

1. Field of Invention

Embodiments of the present invention relate generally to electronic devices and, more specifically, in one embodiment, to integrators for delta-sigma modulators in electronic devices.

2. Description of Related Art

Generally, memory devices include an array of memory elements and associated sense amplifiers. The memory elements store data, and the sense amplifiers read the data from the memory elements. To read data, for example, a current is passed through the memory element, and the current or a resulting voltage is measured by the sense amplifier. Conventionally, the sense amplifier measures the current or voltage by comparing it to a reference current or voltage. Depending on whether the current or voltage is greater than the reference, the sense amplifier outputs a value of one or zero. That is, the sense amplifier quantizes the analog signal from the memory element into one of two logic states.

Many types of memory elements are capable of assuming more than just two states. For example, some memory elements are capable of muti-bit (e.g., more than two state) storage. For instance, rather than outputting either a high or low voltage, the memory element may output four or eight different voltage levels, each level corresponding to a different data value. However, conventional sense amplifiers often fail to distinguish accurately between the additional levels because the difference between the levels (e.g., a voltage difference) in a multi-bit memory element is often smaller than the difference between the levels in a single-bit (i.e., two state) memory element. Thus, conventional sense amplifiers often cannot read multi-bit memory elements. This problem may be increased as high performance multi-bit memory elements become increasingly dense, thereby reducing the size of the memory elements and the difference between the levels (e.g., voltage) to be sensed by the sense amplifiers.

A variety of factors may tend to prevent the sense amplifier from discerning small differences in the levels of a multi-bit memory element. For instance, noise in the power supply, ground, and reference voltage may cause an inaccurate reading of the memory element. The noise may have a variety of sources, such as temperature variations, parasitic signals, data dependent effects, and manufacturing process variations. This susceptibility to noise often leads a designer to reduce the number of readable states of the memory element, which tends to reduce memory density and increase the cost of memory.

Conventional sense amplifiers present similar problems in imaging devices. In these devices, an array of light sensors output a current or voltage in response to light impinging upon the sensor. The magnitude of the current or voltage typically depends upon the intensity of the light. Thus, the capacity of the sense amplifier to accurately convert the current or voltage into a digital signal may determine, in part, the fidelity of the captured image. Consequently, noise affecting the sense amplifier may diminish the performance of imaging devices.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an electronic device in accordance with an embodiment of the present invention;

FIG. 2 illustrates a memory device in accordance with an embodiment of the present invention;

FIG. 3 illustrates a memory array in accordance with an embodiment of the present invention;

FIG. 4 illustrates a memory element in accordance with an embodiment of the present invention;

FIG. 5 illustrates I-V traces of memory elements storing different values, in accordance with an embodiment of the present invention;

FIG. 6 illustrates noise in the bit-line current during a read operation;

FIG. 7 illustrates a quantizing circuit in accordance with an embodiment of the present invention;

FIG. 8 illustrates a delta-sigma sensing circuit in accordance with an embodiment of the present invention;

FIGS. 9 and 10 illustrate current flow during operation of the quantizing circuit of FIG. 8;

FIGS. 11-13 illustrate voltages in the quantizing circuit of FIG. 8 when sensing small, medium, and large currents, respectively;

FIG. 14 is a graph of bit-line current versus counter output for the quantizing circuit of FIG. 8;

FIG. 15 is a graph of count versus quantizing circuit output in accordance with an embodiment of the present invention;

FIG. 16 is a block diagram of a delta-sigma modulator in accordance with an embodiment of the present invention;

FIG. 17 is a block diagram of a one-bit delta-sigma modulator in accordance with an embodiment of the present invention;

FIG. 18 illustrates an integrator in accordance with an embodiment of the present invention; and

FIG. 19 illustrates an example of a system including the memory device of FIG. 2.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Various embodiments of the present invention are described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

Some of the subsequently described embodiments may address one or more of the problems with conventional sense amplifiers discussed above. Some embodiments include a quantizing circuit configured to detect small differences in voltages and/or currents. As explained below, the quantizing circuit may sample the measured electrical parameter on multiple occasions and filter, e.g., average or sum, the samples to reduce the impact of noise. As a result, in some embodiments, the quantizing circuit may resolve small differences between voltage or current levels in multi-bit memory elements and/or light sensors, which may allow circuit designers to increase the number of bits stored per memory element and/or the sensitivity of an imaging device.

FIG. 1 depicts an electronic device 10 that may be fabricated and configured in accordance with one or more of the present embodiments. The illustrated electronic device 10 includes a memory device 12 that, as explained further below, may include multi-bit memory elements and quantizing circuits. Alternatively, or additionally, the electronic device 10 may include an imaging device 13 having the quantizing circuits.

Myriad devices may embody one or more of the present techniques. For example, the electronic device 10 may be a storage device, a communications device, an entertainment device, an imaging system, or a computer system, such as a personal computer, a server, a mainframe, a tablet computer, a palm-top computer, or a laptop.

FIG. 2 depicts a block diagram of an embodiment of the memory device 12. The illustrated memory device 12 may include a memory array 14, a quantizing circuit 16, a column decoder 18, a column address latch 20, row drivers 22, a row decoder 24, row address latches 26, and control circuitry 28. As described below with reference to FIG. 3, the memory array 14 may include a matrix of memory elements arranged in rows and columns. As will be appreciated, the imaging device 13 (FIG. 1) may include similar features except that in the case of an imaging device 13, the array 14 might comprise an array of imaging elements, such as complementary-metal-oxide semiconductor (CMOS) imaging elements or charge coupled devices (CCDs).

When accessing the memory elements, the control circuitry may receive a command to read from or write to a target memory address. The control circuitry 28 may then convert the target address into a row address and a column address. In the illustrated embodiment, the row address bus 30 transmits the row address to the row address latches 26, and a column address bus 32 transmits column address to the column address latches 20. After an appropriate settling time, a row address strobe (RAS) signal 34 (or other controlling clock signal) may be asserted by the control circuitry 28, and the row address latches 26 may latch the transmitted row address. Similarly, the control circuitry 28 may assert a column address strobe 36, and the column address latches 20 may latch the transmitted column address.

Once row and column addresses are latched, the row decoder 24 may determine which row of the memory array 14 corresponds to the latched row address, and the row drivers 22 may assert a signal on the selected row. Similarly, the column decoder 18 may determine which column of the memory array 14 corresponds with the latched column address, and the quantizing circuit 16 may quantize (e.g., sense) a voltage or current on the selected column. Additional details of reading and writing are described below.

FIG. 3 illustrates an example of a memory array 14. The illustrated memory array 14 includes a plurality of bit-lines 38, 40, 42, 44, and 46 (also referred to as BL0-BL4) and a plurality of word-lines 48, 50, 52, 54, 56, 58, 60, and 62 (also referred to as WL0-WL7). These bit-lines and word-lines are examples of electrical conductors. The memory array 14 further includes a plurality of memory elements 64, each of which may be arranged to intersect one of the bit-lines and one of the word-lines. In other embodiments, imaging elements may be disposed at each of these intersections.

The memory elements and imaging elements may be referred to generally as data locations, i.e., devices or elements configured to convey data, either stored or generated by a sensor, when sensed by a sensing circuit, such as the quantizing circuits discussed below. The data locations may be formed on an integrated semiconductor device (e.g., a device formed on a single crystal of silicon) that also includes the other components of the memory device 12 (or imaging device 13).

In some embodiments, the illustrated memory elements 64 are flash memory devices. The operation of the flash memory elements is described further below with reference to the FIGS. 4 and 5. It should be noted that, in other embodiments, the memory elements 64 may include other types of volatile or nonvolatile memory. For example, the memory elements 64 may include a resistive memory, such as a phase change memory or magnetoresistive memory. In another example, the memory elements 64 may include a capacitor, such as a stacked or trench capacitor. Some types of memory elements 64 may include an access device, such as a transistor or a diode associated with each of the memory elements 64, or the memory elements 64 may not include an access device, for instance in a cross-point array.

FIG. 4 illustrates a circuit 66 that models the operation of an arbitrarily selected memory element 64, which is disposed at the intersection of WL3 and BL0. This circuit 66 includes a capacitor 68, a pre-drain resistor 70 (R_(PD)), a post-source resistor 72 (R_(PS)), and a ground 74. The resistors 70 and 72 model the other devices in series with the memory element 64 being sensed. The illustrated memory element 64 includes a gate 76, a floating gate 78, a drain 80, and a source 82. In the circuit 66, the drain 80 and source 82 are disposed in series between the pre-drain resistor 70 and the post-source resistor 72. The gate 76 is connected to WL3. The pre-drain resistor 70, the drain 80, the source 82, and the post-source resistor 72 are disposed in series on the bit-line BL0. The capacitor 68, which models the capacitance of the bit-line, has one plate connected to ground 74 and another plate connected to the bit-line BL0, in parallel with the memory elements 64.

Several of the components of the circuit 66 represent phenomenon affecting the memory elements 64 when it is sensed. The pre-drain resistor 70 generally represents the drain-to-bitline resistance of the memory elements 64 connected to the bit-line above (i.e., up current from) WL3 when these memory elements 64 are turned on, (e.g., during a read operation). Similarly, the post source resistor 72 generally corresponds to the source-to-ground resistance of the memory elements 64 connected to the bit-line below WL3 when the memory element 64 is sensed. The circuit 66 models electrical phenomena associated with reading the memory elements 64 at the intersection of WL3 and BL0.

The operation of the memory elements 64 will now be briefly described with reference to FIGS. 4 and 5. FIG. 5 illustrates one potential relationship between the bit-line current (I_(BIT)), the word-line voltage (V_(WL)), and the voltage of the floating gate 78 (V_(FG)). As illustrated by FIG. 5, V_(FG) affects the response of the memory element 64 to a given V_(WL). Decreasing the voltage of the floating gate shifts the I-V curve of the memory elements 64 to the right. That is, the relationship between the bit-line current and a word-line voltage depends on the voltage of the floating gate 78. The memory elements 64 may store data by exploiting this effect.

To write data to the memory elements 64, a charge corresponding to the data may be stored on the floating gate 78. The charge of the floating gate 78 may be modified by applying voltages to the source 82, drain 80, and/or gate 76 such that the resulting electric fields produce phenomenon like Fowler-Northam tunneling and/or hot-electron injection near the floating gate 78. Initially, the memory elements 64 may be erased by applying a word-line voltage designed to drive electrons off of the floating gate 78. In some embodiments, an entire column or block of memory elements 64 may be erased generally simultaneously. Once the memory elements 64 are erased, the gate 76 voltage may be manipulated to drive a charge onto the floating gate 78 that is indicative of a data value. After the write operation ends, the stored charge may remain on the floating gate 78 (i.e., the memory elements 64 may store data in a nonvolatile fashion).

As illustrated by FIG. 5, the value stored by the memory element 64 may be read by applying a voltage, V_(WL), to the gate 76 and quantizing (e.g., categorizing) a resulting bit-line current, I_(BIT). Each of the I-V traces depicted by FIG. 5 correspond to a different charge stored on the floating gate, V_(FG), which should not be confused with the voltage that is applied to the gate, V_(WL). The difference in floating gate 70 voltage, V_(FG), between each I-V trace is an arbitrarily selected scaling factor “x.” The illustrated I-V traces correspond to eight-different data values stored by the memory element 64, with a V_(FG) of 0x representing a binary data value of 000, a V_(FG) of 1x representing a binary data value of 001, and so on through V_(FG) of 7x, which represents a binary data value of 111. Thus, by applying a voltage to the gate 76 and measuring the resulting bit-line current, the charge stored on the floating gate 78 may be sensed, and the stored data may be read.

The accuracy with which the bit-line current is quantized may affect the amount of data that a designer attempts to store in each memory element 64. For example, in a system with a low sensitivity, a single bit may be stored on each memory element 64. In such a system, a floating gate voltage V_(FG) of 0x may represent a binary value of 0, and a floating gate voltage V_(FG) of −7x may represent a binary value of one. Thus, the difference in floating gate voltages V_(FG) corresponding to different data values may be relatively large, and the resulting differences and bit-line currents for different data values may also be relatively large. As a result, even low-sensitivity sensing circuitry may quantize (e.g., discern) these large differences in bit-line current during a read operation. In contrast, high-sensitivity sensing circuitry may facilitate storing more data in each memory element 64. For instance, if the sensing circuitry can distinguish between the eight different I-V traces depicted by FIG. 5, then the memory elements 64 may store three bits. That is, each of the eight different charges stored on the floating gate 78 may represent a different three-bit value: 000, 001, 010, 011, 100, 101, 110, or 111. Thus, circuitry that precisely quantizes the bit-line current I_(BIT) may allow a designer to increase the amount of data stored in each memory element 64.

However, as mentioned above, a variety of effects may interfere with accurate measurement of the bit-line current. For instance, the position of the memory elements 64 along a bit-line may affect R_(PD) and R_(PS), which may affect the relationship between the word-line voltage V_(WL) and the bit-line current I_(BIT). To illustrate these effects, FIG. 6 depicts noise on the bit-line while reading from the memory element 64. As illustrated, noise in the bit-line current I_(BIT) may cause the bit-line current I_(BIT) to fluctuate. Occasionally, the fluctuation may be large enough to cause the bit-line current I_(BIT) to reach a level that represents a different stored data value, which could cause the wrong value to be read from the memory elements 64. For instance, if the bit-line current is sensed at time 84, corresponding to an arbitrarily selected peak, a data value of 100 may be read rather than the correct data value of 011. Similarly, if the bit-line current is sensed at time 86, corresponding to an arbitrarily selected local minimum, a data value of 010 may be read rather than a data value of 011. Thus, noise on the bit-line may cause erroneous readings from memory elements 64.

FIG. 7 depicts a quantizing circuit 16 that may tend to reduce the likelihood of an erroneous reading. The illustrated quantizing circuit 16 includes an analog-to-digital converter 88 and a digital filter 90 connected to each of the bit-lines 38, 40, 42, 44, and 46, respectively. Each bit-line 38, 40, 42, 44, and 46 may connect to a different analog-to-digital converter 88 and digital filter 90. The digital filters 90, in turn, may connect to an input/output bus 92, which may connect to a column decoder 18, a column address latch 20, and/or control circuitry 28 (see FIG. 2).

In operation, the quantizing circuit 16 may quantize (e.g., digitize) analog signals from the memory elements 64 in a manner that is relatively robust to noise. As explained below, the quantizing circuit 16 may do this by converting the analog signals into a bit-stream and digitally filtering high-frequency components from the bit-stream.

The analog-to-digital converter 88 may be a one-bit, analog-to-digital converter or a multi-bit, analog-to-digital converter. In the present embodiment, an analog-to-digital converter 88 receives an analog signal from the memory element 64, e.g., a bit-line current I_(BIT) or a bit-line voltage V_(BL), and outputs a bit-stream that represents the analog signal. The bit-stream may be a one-bit, serial signal with a time-averaged value that generally represents the time-averaged value of the analog signal from the memory element 64. That is, the bit-stream may fluctuate between values of zero and one, but its average value, over a sufficiently large period of time, may be proportional to the average value of the analog signal from the memory element 64. In certain embodiments, the bit-stream from the analog-to-digital converter 88 may be a pulse-density modulated (PDM) version of the analog signal. The analog-to-digital converter 88 may transmit the bit-stream to the digital filter 90 on a bit-stream signal path 94.

The digital filter 90 may digitally filter high-frequency noise from the bit-stream. To this end, the digital filter 90 may be a low-pass filter, such as a counter, configured to average (e.g., integrate and divide by the sensing time) the bit-stream over a sensing time, i.e., the time period over which the memory element 64 is read. (Alternatively, in some embodiments, the digital filter 90 is configured to integrate the bit-stream without dividing by the sensing time.) As a result, the digital filter 90 may output a value that is representative of both the average value of the bit-stream and the average value of the analog signal from the memory element 64. In some embodiments, the digital filter 90 is a counter, and the cut-off frequency of the digital filter 90 may be selected by adjusting the duration of the sensing time. In the present embodiment, increasing the sensing time will lower the cutoff frequency. That is, the frequency response of the digital filter 90 may be modified by adjusting the period of time over which the bit-stream is integrated and/or averaged before outputting a final value. The frequency response of the digital filter 90 is described further below with reference to FIG. 15. For multi-bit memory elements 64, the output from the digital filter 90 may be a multi-bit binary signal, e.g., a digital word that is transmitted serially and/or in parallel.

Advantageously, in certain embodiments, the quantizing circuit 16 may facilitate the use of multi-bit memory elements 64. As described above, in traditional designs, the number of discrete data values that a memory element 64 stores may be limited by sense amps that react to noise. In contrast, the quantizing circuit 16 may be less susceptible to noise, and, as a result, the memory elements 64 may be configured to store additional data. Without the high frequency noise, the intervals between signals representative of different data values may be made smaller, and the number of data values stored by a given memory element 64 may be increased. Thus, beneficially, the quantizing circuit 16 may read memory elements 64 that store several bits of data, e.g., 2, 3, 4, 5, 6, 7, 8, or more bits per memory element 64.

Although the quantizing circuit 16 may sense the signal from the memory element 64 over a longer period of time than conventional designs, the overall speed of the memory device 12 may be improved. As compared to a conventional device, each read or write operation of the memory device 12 may transfer more bits of data into or out of the memory element 64. As a result, while each read or write operation may take longer, more data may be read or written during the operation, thereby improving overall performance. Further, in some memory devices 12, certain processes may be performed in parallel with a read or write operation, thereby further reducing the overall impact of the longer sensing time. For example, in some embodiments, the memory array 14 may be divided into banks that operate at least partially independently, so that, while data is being written or read from one bank, another bank can read or write data in parallel.

FIG. 8 illustrates details of one implementation of the quantizing circuit 16. In this embodiment, the digital filter 90 is a counter, and the analog-to-digital converter 88 is a first-order delta-sigma modulator. The illustrated delta-sigma modulator 88 may include a latched comparator 96, a capacitor 98, and a switch 100. In other embodiments, other types of digital filters and analog-to-digital converters may be employed, such as those described below in reference to FIGS. 17 and 18.

As illustrated, an input of the counter 90 may connect to the bit-stream signal path 94, which may connect to an output of the comparator 96. The output of the comparator 96 may also connect to a gate of the switch 100 by a feedback signal path 102. The output terminal (e.g., source or drain) of the switch 100 may connect in series to one of the bit-lines 38, 40, 42, 44, or 46, and the input terminal of the switch 100 may connect to a reference current source 104 (I_(REF)). One plate of the capacitor 98 may connect to one of the bit-lines 38, 40, 42, 44, or 46, and the other plate of the capacitor 98 may connect to ground.

The illustrated counter 90 counts the number of clock cycles that the bit-stream 94 is at a logic high value or logic low value during the sensing time. The counter may count up or count down, depending on the embodiment. In some embodiments, the counter 90 may do both, counting up one for each clock cycle that the bit-stream has a logic high value and down one for each clock cycle that the bit-stream has a logic low value. Output terminals (D0-D5) of the counter 90 may connect to the input/output bus 92 for transmitting the count. The counter 90 may be configured to be reset to zero or some other value when a reset signal is asserted. In some embodiments, the counter 90 may be a series connection of D-flip flops, e.g., D-flip flops having SRAM or other memory for storing an initial value and/or values to be written to the memory element 64.

In the illustrated embodiment, the clocked comparator 96 compares a reference voltage (V_(REF)) to the voltage of one of the bit-lines 38, 40, 42, 44, or 46 (V_(BL)), which may be generally equal to the voltage of one plate of the capacitor 98. The comparator 96 may be clocked (e.g., falling and/or rising edge triggered), and the comparison may be performed at regular intervals based on the clock signal, e.g., once per clock cycle. Additionally, the comparator 96 may latch, i.e., continue to output, values (V_(FB)) between comparisons. Thus, when the clock signals the comparator 96 to perform a comparison, if V_(BL) is less than V_(REF), then the comparator 96 may latch its output to a logic low value, as described below in reference to FIG. 9. Conversely, if V_(BL) is greater than V_(REF), then the comparator 96 may latch a logic high value on its output, as described below in reference to FIG. 10. As a result, the illustrated comparator 96 outputs a bit-stream that indicates whether V_(BL) is larger than V_(REF), where the indication is updated once per clock cycle.

Advantageously, in some embodiments, the quantizing circuit 16 may include a single comparator (e.g., not more than one) for each column of multi-level memory elements 64. In contrast, conventional sense amplifiers often include multiple comparators to read from a multi-bit memory cell, thereby potentially increasing device complexity and cost.

The capacitor 98 may be formed by capacitive coupling of the bit-lines 38, 40, 42, 44, and 46. In other designs, this type of capacitance is referred to as parasitic capacitance because it often hinders the operation of the device. However, in this embodiment, the capacitor 98 may be used to integrate differences between currents on the bit-lines 38, 40, 42, 44, or 46 and the reference current to form the bit-stream, as explained further below. In some embodiments, the capacitor 98 may be supplemented or replaced with an integrated capacitor that provides greater capacitance than the “parasitic” bit-line capacitance.

The illustrated switch 100 selectively transmits current I_(REF) from the reference current source 104. In various embodiments, the switch 100 may be a PMOS transistor (as illustrated in FIGS. 8-10) or an NMOS transistor (as illustrated in FIG. 17) controlled by the V_(FB) signal on the feedback signal path 102.

The operation of the quantizing circuit 16 will now be described with reference to FIGS. 9-12. Specifically, FIGS. 9 and 10 depict current flows in the quantizing circuit 16 when the comparator 96 is latched low and high, respectively. FIG. 11 illustrates V_(BL), the bit-stream output from the comparator 96, and the corresponding increasing count of the counter 90 for a relatively small bit-line current. FIG. 12 depicts the same voltages when measuring a medium sized bit-line current, and FIG. 13 depicts these voltages when measuring a relatively large bit-line current.

To sense the current through the memory element 64, the illustrated delta-sigma modulator 88 exploits transient effects to output a bit-stream representative of the bit-line current I_(BIT). Specifically, the delta-sigma modulator 88 may repeatedly charge and discharge the capacitor 98 with a current divider that subtracts the bit-line current I_(BIT) from the reference current I_(REF). Consequently, a large current through the memory element 64 may rapidly discharge the capacitor 98, and a small current through the memory element 64 may slowly discharge the capacitor 98.

To charge and discharge the capacitor 98, the delta-sigma modulator 88 switches between two states: the state depicted by FIG. 9 (hereinafter “the charging state”) and the state depicted by FIG. 10 (hereinafter “the discharging state”). Each time the delta-sigma modulator 88 transitions between these states, the bit-stream changes from a logic high value to a logic low value or vice versa. The proportion of time that the delta-sigma modulator 88 is in the state illustrated by either FIG. 9 or FIG. 10 may be proportional to the size of the bit-line current I_(BIT) through the memory element 64. The larger the bit-line current I_(BIT), the more time that the delta-sigma modulator 88 is in the state illustrated by FIG. 9, rather than the state illustrated by FIG. 10, and the more time that the bit-stream has a logic low value.

Starting with the charging state (FIG. 9), the capacitor 98 may initially accumulate a charge (e.g., become more charged). To this end, the output of the comparator 96 is latched to logic low, which, as mentioned above, may occur when V_(BL) is less than V_(REF). The logic low may be conveyed to switch 100 by the feedback signal path 102, and the switch 100 may close, thereby conducting the reference current I_(REF) through one of the bit-lines 38, 40, 42, 44, or 46, as indicated by the larger arrows in FIG. 9. A portion of the electrons flowing through the reference current source 104 may be accumulated by the capacitor 98, as indicated by the smaller-horizontal arrows, and the remainder may be conducted through the memory element 64, i.e., the bit-line current I_(BIT), as indicated by the smaller vertical arrows. Thus, the capacitor 98 may accumulate a charge, and V_(BL) may increase.

The comparator 96 and the reference current source 104 may cooperate to charge the capacitor 98 for a discrete number of clock cycles. That is, when the delta-sigma modulator 88 transitions to the charging state, the delta-sigma modulator 88 may remain in this state for an integer number of clock cycles. In the illustrated embodiment, the comparator 96, the output of which is latched, changes state no more than once per clock cycle, so the switch 100, which is controlled by the output of the comparator 96, V_(FB), conducts current for a discrete number of clock cycles. As a result, the reference current source 104 conducts current I_(REF) through the bit-line and into the capacitor 98 for an integer number of clock cycles.

After each clock cycle of charging the capacitor 98, the delta-sigma modulator 88 may transition from the charging state to the discharging state, which is illustrated by FIG. 10, depending on the relative values of V_(BL) and V_(REF). Once per clock cycle (or at some other appropriate interval, such as twice per clock cycle), the comparator 96 may compare the voltage of the capacitor V_(BL) to the reference voltage V_(REF). If the capacitor 98 has been charged to the point that V_(BL) is greater than V_(REF), then the output of the comparator 96 may transition to logic high, as illustrated in FIG. 10. The logic high signal may be conveyed to the switch 100 by the feedback signal path 102, thereby opening the switch 100. As a result, the reference current source 104 may cease conducting current through the memory element 64 and into the capacitor 98, and the capacitor 98 may begin to discharge through the memory element 64.

In the present embodiment, the delta-sigma modulator 88 discharges the capacitor 98 for a discrete number of clock intervals. After each clock cycle of discharging the capacitor 98, the delta-sigma modulator 88 compares V_(BL) to V_(REF). If V_(BL) is still greater than V_(REF), then the comparator 96 may continue to output a logic high signal, i.e., V_(FB)=1, and the switch 100 remains open. On the other hand, if enough current has flowed out of the capacitor 98 that V_(BL) is less than V_(REF), then the comparator 96 may output a logic low signal, i.e., V_(FB)=0, and the switch 100 may close, thereby transitioning the delta-sigma modulator 88 back to the charging state and initiating a new cycle.

The counter 90 may count the number of clock cycles that the delta-sigma modulator 88 is in either the charging state or the discharging state by monitoring the bit-stream signal path 94. The bit-stream signal path 94 may transition back and forth between logic high and logic low with the output of the comparator 96, V_(FB), and the counter 90 may increment and/or decrement a count once per clock cycle (or other appropriate interval) based on whether the bit-stream is logic high or logic low. After the sensing time has passed, the counter 90 may output a signal indicative of the count on output terminals D0-D5. As explained below, the count may correspond, e.g., proportionally, to the bit-line current, I_(BIT).

FIGS. 11-13 illustrate voltages V_(FB) and V_(BL) in the quantizing circuit 16 when reading data from a memory element 64. Specifically, FIG. 11 illustrates a low-current case, in which the value stored by the memory element 64 is represented by a relatively low bit-line current. Similarly, FIG. 12 illustrates a medium-current case, and FIG. 13 illustrates a high-current case. In each of these figures, the ordinate of the lower trace represents the voltage of the bit-stream signal path 94, V_(FB), and the ordinate of the upper trace illustrates the bit-line voltage, V_(BL). The abscissa in each of the traces represents time, with the lower trace synchronized with the upper trace, and the duration of the time axes is one sensing time 106.

As illustrated by FIG. 11, the counter 90 is initially preset to zero (or some other appropriate value) by applying a reset signal. In some embodiments, the delta-sigma modulator 88 may undergo a number of start-up cycles to reach steady-state operation before initiating the sensing time and resetting the counter 90. At the beginning of the illustrated read operation, the delta-sigma modulator 88 is in the charging state, which charges the capacitor 98 and increases V_(BL), as indicated by dimension arrow 108. At the beginning of the next clock cycle, the comparator 96 compares the bit-line voltage to the reference voltage and determines that the bit-line voltage is greater than the reference voltage. As a result, the bit-stream signal path 94 (V_(FB)) transitions to a logic high voltage, and the delta-sigma modulator 88 transitions to the discharging state. Additionally, the counter 90 increments the count by one to account for one clock cycle of the bit-stream signal 94 holding a logic low value. Next, the charge stored on the capacitor 98 drains out through the memory element 64, and the bit-line voltage drops until the comparator 96 determines that V_(BL) is less than V_(REF), at which point the cycle repeats. The cycle has a period 112, which may be divided into a charging portion 114 and a discharging portion 116. Once during each cycle in the sensing time 106, the count stored in the counter 90 may increase by one. At the end of the sensing time 106, the counter 90 may output the total count.

A comparison of FIG. 11 to FIGS. 12 and 13 illustrates why the count correlates with the bit-line current. In FIG. 13, the high-current case, the stored charge drains from the capacitor 98 quickly, relative to the other cases, because the bit-line current I_(BIT) is large and, as a result, the delta-sigma modulator 88 spends more time in the charging state than the discharging state. As a result, the bit-stream has a logic low value for a large portion of the sensing time 106, thereby increasing the count.

The capacitance of the capacitor 98 may be selected with both the clock frequency and the range of expected bit-line currents in mind. For example, the capacitor 98 may be large enough that the capacitor 98 does not fully discharge (e.g., saturate) when the bit-line current I_(BIT) is either at its lowest expected value or at its highest expected value. That is, in some embodiments, the capacitor 98 generally remains in a transient state while reading the memory element 64. Similarly, the frequency at which the comparator 96 is clocked may affect the design of the capacitor 98. A relatively high frequency clock signal may leave the capacitor 98 with relatively little time to discharge or saturate between clock cycles, thereby leading a designer to choose a smaller capacitor 98.

Similarly, the size of the reference current may be selected with the range of expected bit-line currents in mind. Specifically, in certain embodiments, the reference current is less than the largest expected bit-line current I_(BIT), so that, in the case of maximum bit-line current I_(BIT), the capacitor 98 can draw charge from the reference current while the rest of the reference current flows through the memory element 64.

FIG. 14 illustrates the relationship between the bit-line current I_(BIT) and the count for the presently discussed embodiment. As illustrated by FIG. 14, the count corresponds with (e.g., is generally proportional to) the bit-line current I_(BIT). This relationship is described by the following equation (Equation 1), in which N_(ST) represents the number of clock cycles during the sensing time:

I _(BIT) /I _(REF)=Count/N _(ST)

Thus, in the illustrated embodiment, the count corresponds with (e.g., is indicative of) the bit-line current I_(BIT), which corresponds with the value stored by the memory element 64.

Advantageously, the quantizing circuit 16 may quantize (e.g., categorize) the bit-line current I_(BIT) as falling into one of a large number of categories, each of which is represented by an increment of the count. In doing so, in some embodiments, the quantizing circuit 16 may resolve small differences in the bit-line current I_(BIT). The resolution of the quantizing circuit 16 may be characterized by the following equation (Equation 2), in which I_(MR) represents the smallest resolvable difference in bit-line current I_(BIT), i.e., the resolution of the quantizing circuit 16:

I _(MR) =I _(REF) /N _(ST)

Thus, the resolution of the quantizing circuit 16 may be increased by increasing the sensing time or the clock frequency or by decreasing I_(REF), which may limit the maximum cell current since I_(MR) is less than I_(REF).

The resolution of the quantizing circuit 16 may facilitate storing multiple bits in the memory element 64 or sensing multiple levels of light intensity in an image sensor element. For example, if the quantizing circuit 16 is configured to quantize (e.g., categorize) the bit-line current I_(BIT) into one of four different levels, then the memory element 64 may store two-bits of data or, if the quantizing circuit 16 is configured to categorize the bit-line current I_(BIT) into one of eight different current levels, then the memory element 64 may store three-bits of data. For the present embodiment, the number of bits stored by the memory element 64 may be characterized by the following equation (Equation 3), in which N_(B) represents the number of bits stored by a memory element 64 and I_(RANGE) represents the range of programmable bit-line currents through the memory element 64:

N _(B)=log (I _(RANGE) /I _(MR))/log 2

In short, in the present embodiment, greater resolution translates into higher density data storage for a given memory element 64.

FIG. 15 is a graph that illustrates one way in which the counter 90 may be configured to further reduce the effects of noise. In FIG. 15, the abscissa represents the count, and the ordinate represents the output of the quantizing circuit 16. In the present embodiment, the three-least-significant digits of the count are disregarded as potentially corrupted by noise. That is, D0-D2 (FIG. 8) either do not connect to the input/output bus 92 or are not interpreted as conveying data that is stored by the memory element 64. As a result, a range of counter values may represent a single data value stored by the memory element 64. For example, in the present embodiment, count values ranging from 00 1000 to 00 1111 are construed as representing a data value of 001. Representing data in this manner may further reduce the effects of noise because, even if noise affects the count, in many embodiments, it would have to affect the count in a consistent manner over a substantial portion of the sensing time to affect the more significant digits of the count. That is, disregarding less significant digits may lower the cutoff frequency of the counter 90. In other embodiments, fewer, more, or no digits may be truncated from the count as potentially representing noise.

Truncating less significant digits may introduce a rounding error, or a downward bias, in the output. This effect may be mitigated by presetting (e.g., driving latches to a particular state in advance of counting or storing a value in memory) the counter 90 in a manner that accounts for this bias. The counter 90 may be preset either before reading from the memory element 64 or before writing to the memory element 64. In some embodiments, the preset value may be one-half of the size of the range of counter values that represent a single output value. In other words, if m digits are truncated from the output, then the counter 90 may be preset to one-half of 2^(m) before reading from a memory element 64 or before writing to the memory element 64. In some embodiments, the memory in the counter 90 may store this preset value.

Delta-sigma modulators may be embodied by a variety of circuit topologies, including the one illustrated by FIG. 8. Many of these topologies are depicted more generically by FIG. 16, which is a block diagram of an example of a first-order delta-sigma modulator 120. The illustrated delta-sigma modulator 120 includes an adder 122, an integrator 124, an analog-to-digital converter 126, and a digital-to-analog converter 128. The illustrated adder 122 receives an analog input signal 130 and a feedback signal 132 from the digital-to-analog converter 128. The illustrated adder 122 outputs a delta signal 134 to an input of the integrator 124, which outputs a sigma signal 136 to an input of the analog-to-digital converter 126. The analog-to-digital converter 126 also receives a reference signal 138. The analog-to-digital converter 126 outputs a digital output signal 140, which is received by an input to the digital-to-analog converter 128.

Less generically but still depicting a variety of topologies, FIG. 17 is a block diagram of an example of a one-bit delta-sigma modulator 142, which may embody the delta-sigma modulator 120 illustrated by FIG. 16, and which may be embodied by the delta-sigma modulator 88 illustrated by FIG. 8. In this example, the integrator 124 is a capacitor and the analog-to-digital converter 126 is a comparator. The reference signal 138 is a voltage V_(REF), and the digital-to-analog converter 128 includes a switch 144 and a reference current source 146.

In operation, the illustrated delta-sigma modulators 120 and 142 sense the analog input signal 130 by integrating a difference between the analog input signal 130 and the feedback signal 132 and exercising feedback control over this integrated difference. The smaller the difference, the stronger or the more frequent the feedback signal 132. For instance, in the embodiment of FIG. 8, the difference between the bit-line current I_(BIT) and the reference current I_(REF) is integrated by the voltage of the capacitor 98, and the comparator 96 controls this voltage by outputting feedback 102, which is converted to an analog feedback signal by the current switch 100 and reference current source 104. In certain embodiments, if the difference between the analog feedback signal 132 (FIGS. 16 and 17) and the analog input signal 130 is accurately measured and integrated, the proportion of time that the feedback signal 132 is applied (and the proportion of time that the digital output 140 is logic high or low) is indicative of the analog input signal 130. Thus, consistently integrating this difference may improve the correlation between the digital output 140 and the analog input 130, thereby potentially improving the accuracy of the quantizing circuit 16.

Several phenomena may prevent the delta-sigma modulators 120 and 142 from accurately integrating the difference between the analog input signal 130 and the analog feedback signal 132. For example, if the adder 122 overloads the digital-to-analog converter 128, the analog feedback signal 132 may not accurately reflect the digital output 140, and the difference being integrated may be inaccurate. In another example, the sigma signal 136 output from the integrator 124 may saturate the integrator 124 (i.e., the sigma signal 136 may reach a maximum or minimum of the integrator 124). For instance, in the delta-sigma modulator 88 of FIG. 8, the bit-line voltage V_(BL) may drop to ground, thereby preventing the capacitor 98 from integrating the difference between the bit-line current I_(BIT) and the reference current I_(REF) by storing a charge. Similarly, the voltage of the bit-line V_(BL) may rise to the voltage source of the reference current source 104 (e.g., V_(DD)), and the reference current I_(REF) may stop flowing, thereby preventing the capacitor 98 from integrating a difference between the reference current I_(REF) and the bit-line current I_(BIT). In some embodiments, the correspondence between the digital output 140 and the analog input 130 may be weakened when the integrator 124 (FIGS. 16 and 17) is not integrating, and the accuracy of the quantizing circuit 16 may be compromised.

FIG. 18 illustrates an example of a delta-sigma modulator 146 with an integrator 148 that may have a wider range than other designs. The illustrated integrator 148 includes a differential amplifier 150 and a capacitor 152. The non-inverting input of the differential amplifier 150 is connected to the reference voltage V_(REF), and the inverting input is connected to the bit-line voltage V_(BL). The output of the differential amplifier 150 is connected to an input of the comparator 96. The plates of the capacitor 152 are each connected to either the bit-line or the output of the differential amplifier 150.

In operation, the integrator 148 may integrate the difference between the bit-line voltage V_(BL) and the reference voltage V_(REF), while holding the bit-line voltage V_(BL) generally constant. In the present embodiment, if the bit-line voltage V_(BL) rises above the reference voltage V_(REF), the output of the differential amplifier V_(C2) falls negative at a generally linear rate, thereby both charging the capacitor 152 and counteracting the change in the bit-line voltage V_(BL) via the capacitor 152. On the other hand, if the bit-line voltage V_(BL) drops below the reference voltage V_(REF), the output of the differential amplifier V_(C2) rises at a generally linear rate, thereby both discharging the capacitor 152 and counteracting the change in the bit-line voltage V_(BL). Thus, in the present embodiment, the voltage of the output of the differential amplifier V_(C2) represents the integral of the difference between the bit-line voltage V_(BL) and the reference voltage V_(REF) over time. The operation of the integrator 148 may be characterized by the following equation (Equation 5), in which t represents time, C₂ represents the capacitance of the capacitor 152, and V_(C2(t=0)) represents the voltage of the output of the differential amplifier V_(C2) at time zero:

$V_{C\; 2} = {{\int_{o}^{t}\frac{\left( {V_{BL} + V_{Ref}} \right)}{C_{2}}} + V_{C\; 2{({t = 0})}}}$

The comparator 96 may control the output of the differential amplifier V_(C2) relative to the reference voltage V_(REF) by attempting to keep V_(C2)>V_(REF) or by attempting to keep V_(C2)<V_(REF). In the illustrated embodiment, when V_(C2) drops below the reference voltage V_(REF), the output of the comparator 96 transitions to a logic low value, and the current switch 100 turns on, thereby conducting the reference current I_(REF) through the bit-line. The reference current I_(REF) may cause the bit-line voltage V_(BL) increase, which the differential amplifier 150 may counteract by increasing its output V_(C2) and charging the capacitor 152. As a result, V_(C2) increases. On the other hand, when the comparator 96 detects that V_(C2) is greater than the reference voltage V_(REF), its output may transition to logic high, thereby stopping the flow of the reference current I_(REF) and causing the bit-line voltage V_(BL) to decrease, which the differential amplifier 150 counteracts by lowering its output voltage V_(C2) and discharging the capacitor 152. Thus, V_(C2) decreases until it drops below V_(REF) and the cycle repeats. As a result, in the present embodiment, the integrator 148 integrates the difference between the bit-line voltage V_(BL) and the reference voltage V_(REF), and the comparator 96 outputs a bit-stream 94 (or the digital output 140 of FIGS. 16 and 17) that is a pulse-density modulated representation of the bit-line current I_(BIT).

Advantageously, the present embodiment may decouple the bit-line voltage V_(BL) from the measurement of the bit-line current I_(BIT). As a result, in some embodiments, the design of the delta-sigma modulator 146 may not be constrained by the capacitance of the capacitor 98, which may be a parasitic capacitance subject to the physical dimensions of the bit-line. The range of the integrator 148 may be selected with the range of potential bit-line currents I_(BIT) in mind, and the risk of the integrator 148 saturating may be reduced, which may tend to improve the reliability and accuracy of the delta-sigma modulator 146.

In some embodiments, the capacitor 152 may be connected to a reset transistor, with one terminal of the reset transistor connected to one plate of the capacitor 152 and the other terminal of the reset transistor connected to the other plate of the capacitor 152. The gate of the reset transistor may be controlled by a reset signal. In this embodiment, the capacitor 152 may be discharged, for example between reading from memory elements 64, by asserting the reset signal. Advantageously, resetting the capacitor 152 may initialize the delta-sigma modulator 146 to a known state, so that the delta-sigma modulator 146 reaches steady-state operation within a predictable period of time.

FIG. 19 depicts an example of a processor-based system 310 that includes the memory device 12 (FIG. 2). Alternatively or additionally, the system 310 may include the imaging device 13. The system 310 may be any of a variety of types such as a computer, pager, cellular phone, personal organizer, control circuit, etc. In a typical processor-based system, one or more processors 312, such as a microprocessor, control the processing of system functions and requests in the system 310. The processor 312 and other subcomponents of the system 310 may include quantizing circuits, such as those discussed above.

The system 310 typically includes a power supply 314. For instance, if the system 310 is a portable system, the power supply 314 may advantageously include a fuel cell, permanent batteries, replaceable batteries, and/or rechargeable batteries. The power supply 314 may also include an AC adapter, so the system 310 may be plugged into a wall outlet, for instance. The power supply 314 may also include a DC adapter such that the system 310 may be plugged into a vehicle cigarette lighter, for instance.

Various other devices may be connected to the processor 312 depending on the functions that the system 310 performs. For instance, a user interface 316 may be connected to the processor 312. The user interface 316 may include buttons, switches, a keyboard, a light pen, a mouse, a digitizer and stylus, and/or a voice recognition system, for instance. A display 318 may also be connected to the processor 312. The display 318 may include an LCD, an SED display, a CRT display, a DLP display, a plasma display, an OLED display, LEDs, and/or an audio display, for example. Furthermore, an RF sub-system/baseband processor 320 may also be connected to the processor 312. The RF sub-system/baseband processor 320 may include an antenna that is connected to an RF receiver and to an RF transmitter (not shown). One or more communication ports 322 may also be connected to the processor 312. The communication port 322 may be adapted to be connected to one or more peripheral devices 324 such as a modem, a printer, a computer, or to a network, such as a local area network, remote area network, intranet, or the Internet, for instance.

The processor 312 generally controls the system 310 by implementing software programs stored in the memory. The memory is operably connected to the processor 312 to store and facilitate execution of various programs. For instance, the processor 312 may be connected to the volatile memory 326 which may include Dynamic Random Access Memory (DRAM) and/or Static Random Access Memory (SRAM). The volatile memory 326 is typically large so that it can store dynamically loaded applications and data. As described further below, the volatile memory 326 may be configured in accordance with embodiments of the present invention.

The processor 312 may also be connected to the memory device 12. The memory device 12 may include a read-only memory (ROM), such as an EPROM, and/or flash memory to be used in conjunction with the volatile memory 326. The size of the ROM is typically selected to be just large enough to store any necessary operating system, application programs, and fixed data. Additionally, the non-volatile memory 328 may include a high capacity memory such as a tape or disk drive memory.

The memory device 10 and volatile memory 326 may store various types of software, such as an operating system or office productivity suite including a word processing application, a spreadsheet application, an email application, and/or a database application. These programs may be stored on a variety of tangible machine readable mediums.

While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims. 

1. A delta-sigma modulator, comprising: an adder; an integrator connected to an output of the adder; an analog-to-digital converter connected to an output of the integrator; and a digital-to-analog converter having an input connected to the output of the analog-to-digital converter and having an output connected to the adder.
 2. The delta-sigma modulator of claim 1, wherein the integrator comprises a capacitor.
 3. The delta-sigma modulator of claim 2, wherein the capacitor is coupled between a first input of the analog-to-digital converter and ground.
 4. The delta-sigma modulator of claim 1, wherein the analog-to-digital converter comprises a first input coupled to the output of the integrator and a second input.
 5. The delta-sigma modulator of claim 4, wherein the second input is configured to receive a reference voltage.
 6. The delta-sigma modulator of claim 1, wherein the analog-to-digital converter comprises a comparator.
 7. The delta-sigma modulator of claim 1, wherein the digital-to-analog converter comprises a reference current source.
 8. The delta-sigma modulator of claim 7, wherein the digital-to-analog converter comprises a switch.
 9. A delta-sigma modulator, comprising: an adder; an integrator comprising a differential amplifier, wherein an output of the adder is connected to a first input of the differential amplifier; an analog-to-digital converter connected to an output of the differential amplifier; and a digital-to-analog converter with an input connected to the output of the analog-to-digital converter and with an output connected to the adder.
 10. The delta-sigma modulator of claim 9, wherein the adder comprises a current divider.
 11. The delta-sigma modulator of claim 9, wherein the integrator comprises a capacitor.
 12. The delta-sigma modulator of claim 9, wherein the adder is connected to a flash memory device comprising a floating gate.
 13. A delta-sigma modulator, comprising: an integrator configured to receive a first voltage at a first input of the integrator; and an analog-to-digital converter configured to receive an output from the integrator at a first input of the analog-to-digital converter.
 14. The delta-sigma modulator of claim 13, wherein the integrator comprises a second input configured to receive a reference voltage.
 15. The delta-sigma modulator of claim 14, wherein the analog-to-digital converter comprises a second input configured to receive the reference voltage.
 16. The delta-sigma modulator of claim 13, wherein the analog-to-digital converter comprises a comparator.
 17. The delta-sigma modulator of claim 13, wherein the integrator comprises a differential amplifier.
 18. The delta-sigma modulator of claim 17, wherein the first input of the integrator comprises a first input of the differential amplifier, and wherein the integrator comprises a capacitor coupled in parallel with the comparator, such that the capacitor is coupled between first input of the differential amplifier and an output of the differential amplifier.
 19. The delta-sigma modulator of claim 13, comprising a current switch and a reference current source coupled to the current switch, wherein an output of the analog-to-digital converter is fed back to the current switch.
 20. The delta-sigma modulator of claim 13, comprising a capacitor coupled between the first input of the integrator and ground. 